Process for manufacturing cellulose objects

ABSTRACT

Process for manufacturing cellulose formed objects, whereby a solution of cellulose is formed in the warm state in a tertiary amine N-oxide and, if necessary, water and the formed solution is cooled with air before introducing it into a coagulation bath. Conditioned air is employed for cooling which exhibits a water content of 0.1 to 7 g water vapor per kg dry air and whose relative humidity amounts to less than 85%.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a process for manufacturing cellulose formedobjects, whereby a solution of cellulose is formed in the warm state ina tertiary amine N-oxide and, if necessary, water and the formedsolution is cooled with air before introducing it into a coagulationbath, as well as a yarn of cellulose filaments.

2. Description of the Related Art

Such a process is described in WO 93/19230, whereby the cooling is totake place immediately after the forming. The object of this process isto reduce the stickiness of the freshly extruded formed objects so thata spinneret with a high hole density can be employed for manufacturingcellulose filaments. For cooling, the formed solution is preferablyexposed to a gas stream.

A cooling of the warm formed solution already takes place as the formedsolution leaves the forming tool, for instance a spinneret, in whichtemperatures are typically above 90° C., and reaches into the so-calledair gap. The area between the forming tool and the coagulation bath inwhich the cellulose is precipitated is referred to as the air gap. Thetemperature in the air gap is lower than in the spinneret, but it issignificantly higher than the room temperature due to the heat radiationfrom the spinneret and the warm-up of the air due to the enthalpy flowof the formed objects. Due to the continuous evaporation of water whichis usually used as a coagulation bath, humid warm conditions prevail inthe air gap. The measure proposed in WO 93/19230, that is to cool theformed solution immediately after the forming, results in a more rapidcooling so that the stickiness of the formed solution decreases morerapidly as a result.

SUMMARY OF THE INVENTION

The present invention is based on the objective to improve such aprocess, and in particular to improve the properties of the formedobjects produced herewith, preferably filaments or a filament yarn.

This objective is met by a process for manufacturing cellulose formedobjects whereby a solution of cellulose is formed in the warm state in atertiary amine N-oxide and, if necessary, water and the formed solutionis cooled with air before introducing it into a coagulation bath,whereby conditioned air is employed for cooling which exhibits a watercontent of 0.1 to 7 g water vapor per kg dry air and whose relativehumidity amounts to less than 85%.

The water content of the conditioned air is preferably 0.7 to 4 g watervapor per kg dry air, and more particularly 0.7 to 2 g. The cooling canbe carried out by streaming air, whereby this air is blown against theformed solution or drawn away from it. The drawing away can be carriedout in such way that conditioned air is provided and is drawn throughe.g. a bundle of freshly spun fibers or filaments. A combination ofblowing and drawing away is especially advantageous.

The formed solution can be exposed to the conditioned air throughout theentire pathway up to the point of introduction into the coagulationbath, or only over a portion of this pathway, whereby it is advantageousto carry out the application of air in the first part, i.e. in the areaof the air gap which is immediately adjacent to the forming tool. Theconditioned air should flow at an angle of 0 to 120°, preferably 90°, inrelation to the direction of movement of the formed solution, wherebythe angle of 0° corresponds to a flow opposite to the running directionof the formed solution.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

With the process of the invention, fibers, in particular filaments,films, hollow filaments, membranes, e.g. for applications in dialysis,oxygenation or filtration, can be manufactured in an advantageousfashion. The forming of the solution to a desired cellulose formedobject can be carried out by known spinnerets for manufacturing fibers,slit nozzles or hollow filament nozzles. Subsequent to the forming, i.e.prior to the introduction of the formed solution into the coagulationbath, the formed solution can be drawn.

A yarn of cellulose filaments, produced from a solution of cellulose ina tertiary amine N-oxide, and if necessary water, is characterized inthat the cross-sectional areas of the filaments exhibit a coefficient ofvariation lower than 12%, preferably lower than 10%.

As already described it is advantageous to cool the freshly extrudedformed objects in the air gap, in order to reduce their stickiness inless time. In order to cool at all, the gas stream must by natureexhibit a temperature which is below the temperature of the formedsolution. According to WO 93/19230 a gas stream is employed which has atemperature ranging from -6 to 24° C.

It has been found, however, that not the temperature itself but ratherthe water content of the air and its relative humidity significantlyaffect the properties of the cellulose formed objects. The water contentof air in g water vapor per kg dry air is often also referred to as themixing ratio. In the following, reference to this is simplified by theunit g/kg. Especially during the manufacture of filaments it has beenfound to be important to create climatic conditions as constant aspossible in the air gap, i.e. to eliminate the effect of normalvariations in the ambient climate. Thereby it is particularly importantthat variations in the air humidity are avoided and that the watercontent of the air is low. Even with air conditioning systems seasonalvariations and to some degree daily variations in rooms cannot beadequately suppressed. In addition, the conditioning should be carriedout as uniformly as possible since even small instabilities concerningthe strength and direction of blowing can negatively influence thestrength, elongation, and the titer constancy of the filaments.

The influence of the water content or the mixing ratio is demonstratedduring the filament production, in particular by irregularities in thefilament cross-sections. When cooled with air conditioned to 20° C. anda water content of 14 g/kg and a relative humidity of 94%, thecoefficient of variation of the filament cross-sectional areas amountsto 30% in a yarn with 50 individual filaments. When the water content isreduced to 1.2 g/kg and relative humidity is lowered to 8.5%, thecoefficient of variation is reduced to 5.8% at the same temperature.Even when warmer air is employed, conditioned for instance to 40° C. butwith a lower water content of 3.4 g/kg and a relative humidity of 7.4%,the resulting coefficient of variation is 11.3%, which is consequentlysmaller by a factor of 2.7 than when cooler air with higher humidity isused. According to the invention it is therefore important to carry outa conditioning of the air gap with dry air. The temperature of thecooling air plays a subordinate role in the process.

The invention will be explained and described in the following infurther detail with reference to further examples.

EXAMPLES

The above mentioned examples and also the examples explained in thefollowing were obtained in that a solution of 14 percent by weightViscokraft ELV chemical wood pulp (International Paper Company) with adegree of polymerization of 680, approx. 76 percent by weightN-methylmorpholine-N-oxide (NMMO), a tertiary amine N-oxide, 10 per centwater by weight and 0.14 per cent gallic acid propyl ester by weight asa stabilizer were spun into a filament yarn through a spinneret platewith 50 holes, each with a 130 μm diameter. The filaments formed in thespinneret (T=110° C.) were cooled in an air gap spanning 18 cm. In theair gap air was blown with a velocity of 0.8 m/s perpendicularly to thefilament bundle. The air was blown from one side toward the bundle, andthe homogeneous distribution of the air was obtained via verynarrow-meshed sieves of 10 cm width. The blowing was carried out for 10cm starting at the exit from the nozzle.

The filaments were drawn in the air gap by a factor of 16 and were driedafter passage through a water bath for coagulation and subsequentwashing baths for removal of the NMMO. The drawing speed amounted to 420m/min.

The respective filament bundles obtained were cut 2 timesperpendicularly to the bundle axis at an interval of one meter. Thecross-sectional areas of the filaments were transmitted via a lightmicroscope (magnification 570:1) and a video camera into a computerimage analysis system (Quantimet 970) and evaluated. The area of eachfilament was determined. From the mean of the filament cross-sectionalareas of each examined bundle, whereby two section pictures per bundlewere evaluated, and the standard deviation, the coefficient of variationof the filament cross-sectional area was calculated in per cent as theratio of standard deviation to the mean.

The production of conditioned air proceeded from air at roomtemperature, 21° C., with a water content of 9.2 g/kg and a relativehumidity of 60%, and which was first cleaned by a filter. To increasethe mixture ratio, the air was mixed with air at 80° C. saturated withwater vapor (relative humidity 100%). To obtain a mass flow m(x) ofconditioned air with a water content x, a mass flow m_(u) of ambient airwith a water content x_(u) was mixed with a mass flow ofwater-vapor-saturated air m_(h) with a water content x_(h) according tom(x)=m_(u) +m_(h). The mixture ratio m_(u) :m_(h) is calculated with thefollowing equation: ##EQU1##

The air stream resulting herefrom was subsequently cooled to the desiredtemperature with a heat exchanger. The relative humidity and the watercontent were determined by means of a psychrometer (ALMEMO 2290-2 withpsychrometer sensor AN 846 or humidity/temperature sensor AFH 9646-2).

For reducing the water content, the ambient air was cooled until itreached a relative humidity of 100%. Subsequently a further cooling tookplace and the condensed water was separated. With this procedure the aircould be dried to a water content of approx. 4 g/kg. Subsequently theair was reheated to the desired temperature. The relative humidity andthe water content were measured by means of the psychrometer.

To obtain conditioned air with a water content below 4 g/kg, the air,which was predried beforehand through a condensation process, wasfurther dried using an air dehumidifier (Hunters model 120 KS). Thereheating of the dry air was carried out as well by means of a heatexchanger. The relative humidity and the water content of the air, whichwas dried to a water content below 4 g/kg, was determined by means of amirror cooled dew point measuring device (MICHELL Instruments S4000 RS).

The following tables specify the examined air conditions, characterizedby the temperature (T/°C.), the water content (x/(g/kg)) and therelative humidity (rH/%), and the coefficients of variation of thefilament cross-sectional areas (V/%).

                  TABLE 1    ______________________________________    Examples according to the invention    Example     T/°C.                       x/(g/kg)    rH/% V/%    ______________________________________    1            6     4.7         80   8.1    2            6     1.8         30   5.0    3           10     1.7         22   5.0    4           10     2.3         30   6.1    5           10     3.0         39   6.6    6           10     3.8         50   6.5    7           10     4.8         62   7.7    8           10     5.4         68   8.5    9           10     0.9         11   5.0    10          20     1.2          9   5.8    11          21     1.0          7   5.4    12          21     2.1         14   8.0    13          21     3.1         20   9.8    14          31     2.1          8   8.4    15          40     3.4          7   11.3    ______________________________________

Table I shows clearly that quasi-independently of the temperature of theconditioned air, the lowest coefficients of variation result if theconditioned air exhibits a low water content as in examples 2, 3, 9, 10and 11, in which the coefficient of variation only ranges from 5 to 6%with a water content in each case below 2 g/kg. In these examples therelative humidity was below 30%. When adhering to the conditions of theinvention, the coefficient of variation even at an elevated temperature(example 15) is lower than at significantly lower temperatures outsideof the range of the invention.

                  TABLE II    ______________________________________    Comparison examples    Example     T/°C.                       x/(g/kg)    rH/% V/%    ______________________________________    16           6     5.1         87   16.1    17          10     7.5         97   14.5    18          11     8.0         97   16.8    19          12     8.2         92   20.8    20          12     8.9         100  21.9    21          20     14.0        94   30.0    22          21     9.2         60   23.4    23          21     13.7        89   26.6    24          21     15.4        100  31.6    ______________________________________

Table II illustrates that outside of the range of the invention thecoefficients of variation of the filament cross-sectional areas areabove 14% and even reach values exceeding 30%. Such high fluctuationsare not desired in the manufacture of filament yarn since theynegatively influence the processing into textile flat structures andlead in particular to an uneven dyeing of the flat structure. Also,based on the differing strengths of the individual filaments, and inrelation to the yarn, processing problems may arise. Additionally,examples 16 and 22 show that for the present invention bothrequirements, i.e. a water content below 7 g water vapor per kg dry airand a relative humidity below 85%, must be guaranteed. In example 16 thewater content was in the range claimed but the air exhibited a higherrelative humidity, and a coefficient of variation of 16.1% resultedherefrom. Example 22 demonstrates the conditions of the ambient air at atemperature of 21° C. with a relative humidity of 60% and a watercontent of 9.2 g/kg. In this example the relative humidity is in therange claimed but not the water content, and a coefficient of variationof 23.4% results herefrom. In addition this example illustrates that inorder to achieve an improvement in the textile properties, it is notsufficient to cool with ambient air, and it is not sufficient to carryout a simple blowing with room air which is cooler than the temperaturegenerally prevailing in the air gap.

All patents, publication and references cited in this application areincorporated herein by reference in their entirety.

I claim:
 1. A process for manufacturing cellulose objects, said processcomprising, forming a solution of cellulose in a warm state in atertiary amine N-oxide and, optionally, water; cooling said formedsolution with air; thereafter introducing said solution into acoagulation bath, wherein said air is conditioned air exhibiting a watercontent of 0.1 to 7 g water vapor per kg dry air and a relative humidityof less than 85%.
 2. The process according to claim 1, wherein the watercontent is between 0.7 and 4 g water vapor per kg dry air.
 3. Theprocess according to claim 1, wherein the cooling is carried out withstreaming air, and wherein the air is blown against the formed solutionand/or drawn away therefrom.
 4. The process according to claim 1,wherein the formed solution is subjected to the conditioned airthroughout the entire process up to the step of introducing saidsolution into the coagulation bath.
 5. The process according to claim 1,wherein the formed solution is subjected to the conditioned air over aportion of the process up to the step of introducing said solution intothe coagulation bath.
 6. The process according to claim 5, wherein theformed solution is subjected to the conditioned air in an initial partof the process.
 7. The process according to claim 1, wherein theconditioned air streams at an angle of 0° to 120°, in relation to adirection of movement of the formed solution, and wherein the angle of0° corresponds to a flow opposite to a running direction of the formedsolution.
 8. The process according to claim 1, wherein the formedsolution is drawn before said introducing step into the coagulationbath.
 9. The process according to claim 1, wherein fibers, filaments,films, or membranes are produced from the solution.
 10. The process ofclaim 2, wherein said water content is from about 0.7 to about 2 g watervapor per kg dry air.
 11. The process of claim 7, wherein said angle isabout 90°.
 12. The process of claim 9, wherein said filaments are hollowfilaments.